Data caching is about storing some PHP variables in cache and retrieving it later from cache.
It is also the foundation for more advanced caching features, such as query caching
and page caching.

The following code is a typical usage pattern of data caching, where $cache refers to
a cache component:

// try retrieving $data from cache$data = $cache->get($key);
if ($data === false) {
// $data is not found in cache, calculate it from scratch$data = $this->calculateSomething();
// store $data in cache so that it can be retrieved next time$cache->set($key, $data);
}
// $data is available here

Since version 2.0.11, cache component provides getOrSet() method
that simplifies code for data getting, calculating and storing. The following code does exactly the same as the
previous example:

Data caching relies on the so-called cache components which represent various cache storage,
such as memory, files, databases.

Cache components are usually registered as application components so
that they can be globally configurable
and accessible. The following code shows how to configure the cache application component to use
memcached with two cache servers:

You can then access the above cache component using the expression Yii::$app->cache.

Because all cache components support the same set of APIs, you can swap the underlying cache component
with a different one by reconfiguring it in the application configuration without modifying the code that uses the cache.
For example, you can modify the above configuration to use APC cache:

Yii supports a wide range of cache storage. The following is a summary:

yii\caching\ApcCache: uses PHP APC extension. This option can be
considered as the fastest one when dealing with cache for a centralized thick application (e.g. one
server, no dedicated load balancers, etc.).

yii\caching\DummyCache: serves as a cache placeholder which does no real caching.
The purpose of this component is to simplify the code that needs to check the availability of cache.
For example, during development or if the server doesn't have actual cache support, you may configure
a cache component to use this cache. When an actual cache support is enabled, you can switch to use
the corresponding cache component. In both cases, you may use the same code
Yii::$app->cache->get($key) to attempt retrieving data from the cache without worrying that
Yii::$app->cache might be null.

yii\caching\FileCache: uses standard files to store cached data. This is particularly suitable
to cache large chunk of data, such as page content.

yii\caching\MemCache: uses PHP memcache
and memcached extensions. This option can be considered as
the fastest one when dealing with cache in a distributed applications (e.g. with several servers, load
balancers, etc.)

yii\redis\Cache: implements a cache component based on Redis key-value store
(redis version 2.6.12 or higher is required).

Tip: You may use different cache storage in the same application. A common strategy is to use memory-based
cache storage to store data that is small but constantly used (e.g. statistical data), and use file-based
or database-based cache storage to store data that is big and less frequently used (e.g. page content).

Note: Do not cache a false boolean value directly because the get() method uses
false return value to indicate the data item is not found in the cache. You may put false in an array and cache
this array instead to avoid this problem.

Some cache storage, such as MemCache, APC, support retrieving multiple cached values in a batch mode,
which may reduce the overhead involved in retrieving cached data. The APIs multiGet()
and multiAdd() are provided to exploit this feature. In case the underlying cache storage
does not support this feature, it will be simulated.

Because yii\caching\Cache implements ArrayAccess, a cache component can be used like an array. The following
are some examples:

Each data item stored in cache is uniquely identified by a key. When you store a data item in cache,
you have to specify a key for it. Later when you retrieve the data item from cache, you should provide
the corresponding key.

You may use a string or an arbitrary value as a cache key. When a key is not a string, it will be automatically
serialized into a string.

A common strategy of defining a cache key is to include all determining factors in terms of an array.
For example, yii\db\Schema uses the following key to cache schema information about a database table:

As you can see, the key includes all necessary information needed to uniquely specify a database table.

Note: Values stored in cache via multiSet() or multiAdd() can
have only string or integer keys. If you need to set more complex key store the value separately via
set() or add().

When the same cache storage is used by different applications, you should specify a unique cache key prefix
for each application to avoid conflicts of cache keys. This can be done by configuring the yii\caching\Cache::$keyPrefix
property. For example, in the application configuration you can write the following code:

A data item stored in a cache will remain there forever unless it is removed because of some caching policy
enforcement (e.g. caching space is full and the oldest data are removed). To change this behavior, you can provide
an expiration parameter when calling set() to store a data item. The parameter
indicates for how many seconds the data item can remain valid in the cache. When you call
get() to retrieve the data item, if it has passed the expiration time, the method
will return false, indicating the data item is not found in the cache. For example,

// keep the data in cache for at most 45 seconds$cache->set($key, $data, 45);
sleep(50);
$data = $cache->get($key);
if ($data === false) {
// $data is expired or is not found in the cache
}

Since 2.0.11 you may set defaultDuration value in your cache component configuration if you prefer a custom cache duration
over the default unlimited duration.
This will allow you not to pass custom duration parameter to set() each time.

Besides expiration setting, cached data item may also be invalidated by changes of the so-called cache dependencies.
For example, yii\caching\FileDependency represents the dependency of a file's modification time.
When this dependency changes, it means the corresponding file is modified. As a result, any outdated
file content found in the cache should be invalidated and the get() call
should return false.

Cache dependencies are represented as objects of yii\caching\Dependency descendant classes. When you call
set() to store a data item in the cache, you can pass along an associated cache
dependency object. For example,

// Create a dependency on the modification time of file example.txt.$dependency = new \yii\caching\FileDependency(['fileName' => 'example.txt']);
// The data will expire in 30 seconds.// It may also be invalidated earlier if example.txt is modified.$cache->set($key, $data, 30, $dependency);
// The cache will check if the data has expired.// It will also check if the associated dependency was changed.// It will return false if any of these conditions are met.$data = $cache->get($key);

Note: Avoid using exists() method along with dependencies. It does not check whether
the dependency associated with the cached data, if there is any, has changed. So a call to
get() may return false while exists() returns true.

$result = $db->cache(function($db){
// the result of the SQL query will be served from the cache// if query caching is enabled and the query result is found in the cachereturn$db->createCommand('SELECT * FROM customer WHERE id=1')->queryOne();
});

Info: Some DBMS (e.g. MySQL)
also support query caching on the DB server-side. You may choose to use either query caching mechanism.
The query caching described above has the advantage that you may specify flexible cache dependencies
and are potentially more efficient.

enableQueryCache: whether to turn on or off query caching.
It defaults to true. Note that to effectively turn on query caching, you also need to have a valid
cache, as specified by queryCache.

queryCacheDuration: this represents the number of seconds
that a query result can remain valid in the cache. You can use 0 to indicate a query result should
remain in the cache forever. This property is the default value used when yii\db\Connection::cache()
is called without specifying a duration.

queryCache: this represents the ID of the cache application component.
It defaults to 'cache'. Query caching is enabled only if there is a valid cache application component.

Any SQL queries in the anonymous function will be cached for the specified duration with the specified dependency.
If the result of a query is found valid in the cache, the query will be skipped and the result will be served
from the cache instead. If you do not specify the $duration parameter, the value of
queryCacheDuration will be used instead.

Sometimes within cache(), you may want to disable query caching for some particular queries. You can use
yii\db\Connection::noCache() in this case.

$result = $db->cache(function($db){
// SQL queries that use query caching// do not use query caching for this command$customer = $db->createCommand('SELECT * FROM customer WHERE id=1')->noCache()->queryOne();
// ...return$result;
});

Query caching does not work with query results that contain resource handlers. For example,
when using the BLOB column type in some DBMS, the query result will return a resource
handler for the column data.

Some caching storage has size limitation. For example, memcache limits the maximum size
of each entry to be 1MB. Therefore, if the size of a query result exceeds this limit,
the caching will fail.